Empty interface variadic function parameter behavior with `...`

[einar-hjortdal]

Describe the bug

A function that expects a variadic array of empty interface elements behaves badly when elements are from an array of another type, being spread with ...

P.S. I am also encountering a cgen error on the project I am currently working on, which I think is related to this issue. Due to the complexity of the project, the cgen error is not reproduced in the below snippet. Please, see here

Actually, I figured out the cgen error is caused by this line

Reproduction Steps

module main

import arrays

interface Value {}

fn some_function(values ...Value) {
	println(values)
	for v in values {
		match v {
			string {
				println(v)
			}
			else {
				println('bro?')
			}
		}
	}
}

fn main() {
	country_ids := [
		'AD',
		'AE',
		'AF',
		'AG',
		'AI',
		'AL',
		'AM',
		'AO',
		'AQ',
		'AR',
		'AS',
		'AT',
		'AU',
		'AW',
		'AX',
		'AZ',
		'BA',
		'BB',
		'BD',
		'BE',
	]
	region_id := 'something'
	some_function(...country_ids) // spreading []string
	some_function(...arrays.concat([Value(region_id)], ...country_ids)) // arrays.concat also fails to cast members
}

Expected Behavior

This should either cause a compilation error, or (what I think should be correct) elements should be cast to the empty interface.

Current Behavior

No runtime error, unexpected runtime behavior

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 47f8f34

Environment details (OS name and version, etc.)

v doctor
|V full version      |V 0.4.10 537605a.47f8f34
|:-------------------|:-------------------
|OS                  |linux, "openmamba release 2024.6 for x86_64 (rolling)"
|Processor           |8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
|Memory              |10.37GB/31.16GB
|                    |
|V executable        |/home/einar/.local/lib64/v/v
|V last modified time|2025-05-09 13:43:59
|                    |
|V home dir          |OK, value: /home/einar/.local/lib64/v
|VMODULES            |OK, value: /home/einar/.vmodules
|VTMP                |OK, value: /tmp/v_1000
|Current working dir |OK, value: /home/einar/Documents/projects/vlang/active/peony
|                    |
|Git version         |git version 2.49.0
|V git status        |weekly.2025.17-69-g47f8f347
|.git/config present |true
|                    |
|cc version          |cc (GCC) 14.2.1 20240909
|gcc version         |gcc (GCC) 14.2.1 20240909
|clang version       |N/A
|tcc version         |tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
|tcc git status      |thirdparty-linux-amd64 0134e9b9
|emcc version        |N/A
|glibc version       |ldd (GNU libc) 2.41

[huly-for-github]

Connected to Huly®: V_0.6-22812

[jorgeluismireles]

Seems related with your #24442

[einar-hjortdal]

Maybe it is, but that one triggers a cgen error, this one just returns unknown interface values. Both have to do with arrays.